Wireless communication technology has gained widespread acceptance in recent years. A wireless local access network (WLAN) is a data transmission system to provide location independent network access between computing devices by using radio waves rather than a cable infrastructure. Often, WLANs are implemented as the final link between existing wired network and a group of client computers, giving these users wireless access to the full resources and services of the corporate network across a building or campus setting.
Wireless local area networks have come into greater use, with the advent of the IEEE 802.11 standard. The rate at which wireless networks are being deployed is accelerating along with their size and ubiquity. Wireless networks using access points based on IEEE standard 802.11, commonly referred erroneously to as WiFi comprise a majority of current wireless deployments. The 802.11 standards were implemented to provide reliable and secure wireless connectivity at high data rates. Like all of the IEEE 802 standards, 802.11 standards focus on the bottom two level of the International Organization for Standardization (ISO) model, the physical layer and the data link layer. The data link layer provides and functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the physical layer. In the current context, this data link layer is further subdivided into Media Access Control (MAC) sublayer that manages interaction of devices with a shared medium. Above the MAC sublayer is the media-independent IEEE 802.2 Logical Link Control (LLC) sublayer that deals with addressing and multiplexing on multi-access media.
Wireless transmissions by hosts within proximity of each other can interfere. Therefore, several medium access control (MAC) protocols for wireless networks have been proposed in the past. In general, MAC protocols may be divided into two types: In centralized protocols, a designated host (often referred to as base station or access point) coordinates access to the wireless medium. A node wanting to transmit must wait until permission to transmit is granted by the coordinator node. The mechanisms for requesting and granting such permission may differ in different protocols. Point Coordination Function (PCF) in IEEE 802.11 is an example of the centralized approach.
In distributed protocols, a coordinator is not needed to arbitrate access to the wireless medium. For instance, in the CSMA (carrier sense multiple access) protocol, a node wishing to transmit a packet does so only if it does not hear another on-going transmission. CSMA protocol is fully distributed, since each node independently determines whether to transmit a packet or not. Distributed Coordination Function (DCF) in IEEE 802.11 is an example of the distributed approach.
There are several benefits of using a distributed approach as compared to a centralized approach: In the centralized approach, if a node cannot communicate with the coordinator, then it cannot transmit any packets. On the other hand, with a distributed protocol, if a node cannot communicate with some nodes, it may still be able transmit packets to other nodes. In the centralized approach, the coordinator has the responsibility of keeping track of the state information for nodes on the LAN. In distributed protocols, this overhead can be eliminated. In a centralized approach, it is difficult to use a battery-powered node as the coordinator, since the coordinator will fail if the battery runs out. With failure-prone coordinators, other nodes must be able to reliably detect failure of the coordinator, and elect a new coordinator.
Challenges exist, however, in implementing a distributed allocation of bandwidth in an ad hoc network not having the benefit of a coordinator. Most of the MAC schemes do not result in optimum transmission pattern, which should provide maximum network utilization, when used in multi-hop networks. In order to provide an optimum transmission pattern or structure, the schedule or queue of all active nodes in the entire network should be known. Given the dynamic and distributed nature of ad hoc networks, the information of the entire network is usually unknown before decisions can be made to start accessing the channel.